Some known power dimmers comprise a switch of thyristor or triac type in series with the load, the assembly receiving an AC power supply voltage. A circuit controls the switch. In operation, the switch is turned on (closed) at an intermediate stage between the beginning and the end of each positive and/or negative halfwave of the power supply voltage, by application of a start signal on its gate. Once started, the switch remains on until the end of the halfwave. By varying the time of application of the start signal, it is possible to regulate the power supplied to the load.
The switch control circuit generally comprises a capacitor charged with the AC voltage and a start circuit capable of turning on the switch when the voltage across the capacitor exceeds a threshold. A potentiometer enables the user to control the capacitor charge speed, and thus the switch starting time.
A disadvantage of existing power dimmers is that, for a given setting of the potentiometer, in case of parasitic fluctuations of the amplitude of the AC power supply voltage, the capacitor charge speed varies and the power supplied to the load accordingly also varies. In the case where the load is a lighting device, this may result in visible light intensity variations.